Windows/Mac/Linux: For the first few days after a new Linux release, grabbing an install image can be tough going. Not so if you've already got an alpha, beta, or RC copy—use zsync to grab the necessary updated bits.

The server you're downloading from must also offer zsync comparison files for the image you're downloading—luckily, with the release of 9.10 Karmic Koala, Ubuntu is now doing so, and many other software providers follow suit for big downloads.